*{
   margin: 0;
   padding: 0;
}
body{
   width: 750px;
   margin: 0 auto;
   max-width:750px;
   min-width: 340px;
}
.top_nav{
   margin-top: 20px;
}
.top_nav ul{
   height: 65px;
   overflow: hidden;
   border-top: 2px solid green;
}
.top_nav ul li{
   float: left;
   list-style: none;
   width: 25%;
   text-align: center;
   margin: 10px 4.1%;
   padding: 10px 0;
}
.top_nav ul li:hover{
   color: greenyellow;
   border-bottom: 4px solid green;
}
.banner img{
   width: 100%;
   height: 100%;
   margin-top: 2px;
}
.centent{
   position: relative;
   width: 100%;
   height: 100%;
   margin: 30px 0;
   border-top: 5px solid #666666;
}

.centent h3{
   margin: 5px 0 10px;
}
.centent p {
   margin: 10px 0 30px ;
}
.centent img:last-child{
   position: absolute;
   right: 0;
   bottom: 10px;
}